Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Frequently in industry, there is a need for process components, containers, and pipes that have adequate toughness to process, contain, and transport fluids at cryogenic temperatures, i.e., at temperatures lower than about -40.degree. C. (-40.degree. F.), without failing. This is especially true in the hydrocarbon and chemical processing industries. For example, cryogenic processes are used to achieve separation of components in hydrocarbon liquids and gases. Cryogenic processes are also used in the separation and storage of fluids such as oxygen and carbon dioxide.
Other cryogenic processes used in industry, for example, include low temperature power generation cycles, refrigeration cycles, and liquefaction cycles. In low temperature power generation, the reverse Rankine cycle and its derivatives are typically used to generate power by recovering the cold energy available from an ultra-low temperature source. In the simplest form of the cycle, a suitable fluid, such as ethylene, is condensed at a low temperature, pumped to pressure, vaporized, and expanded through a work-producing turbine coupled to a generator.
There are a wide variety of applications in which pumps are used to move cryogenic liquids in process and refrigeration systems where the temperature can be lower than about -73.degree. C. (-100.degree. F.). Additionally, when combustible fluids are relieved into a flare system during processing, the fluid pressure is reduced, e.g., across a pressure safety valve. This pressure drop results in a concomitant reduction in temperature of the fluid. If the pressure drop is large enough, the resulting fluid temperature can be sufficiently low that the toughness of carbon steels traditionally used in flare systems is not adequate. Typical carbon steel may fracture at cryogenic temperatures.
In many industrial applications, fluids are contained and transported at high pressures, i.e., as compressed gases. Typically, containers for storage and transportation of compressed gases are constructed from standard commercially available carbon steels, or from aluminum, to provide the toughness needed for fluid transportation containers that are frequently handled, and the walls of the containers must be made relatively thick to provide the strength needed to contain the highly-pressurized compressed gas. Specifically, pressurized gas cylinders are widely used to store and transport gases such as oxygen, nitrogen, acetylene, argon, helium, and carbon dioxide, to name a few. Alternatively, the temperature of the fluid can be lowered to produce a saturated liquid, and even subcooled if necessary, so the fluid can be contained and transported as a liquid. Fluids can be liquefied at combinations of pressures and temperatures corresponding to the bubble point conditions for the fluids. Depending on the properties of the fluid, it can be economically advantageous to contain and transport the fluid in a pressurized, cryogenic temperature condition if cost effective means for containing and transporting the pressurized, cryogenic temperature fluid are available. Several ways to transport a pressurized, cryogenic temperature fluid are possible, e.g., tanker truck, train tankcars, or marine transport. When pressurized cryogenic temperature fluids are to be used by local distributors in the pressurized, cryogenic temperature condition, in addition to the aforementioned storage and transportation containers, an alternative method of transportation is a flowline distribution system, i.e., pipes between a central storage area, where a large supply of the cryogenic temperature fluid is being produced and/or stockpiled, and local distributors or users. All of these methods of transportation require use of storage containers and/or pipes constructed from a material that has adequate cryogenic temperature toughness to prevent failure and adequate strength to hold the high fluid pressures.
The Ductile to Brittle Transition Temperature (DBTT) delineates the two fracture regimes in structural steels. At temperatures below the DBTT, failure in the steel tends to occur by low energy cleavage (brittle) fracture, while at temperatures above the DBTT, failure in the steel tends to occur by high energy ductile fracture. Welded steels used in the construction of process components and containers for the aforementioned cryogenic temperature applications and for other load-bearing, cryogenic temperature service must have DBTTs well below the service temperature in both the base steel and the HAZ to avoid failure by low energy cleavage fracture.
Nickel-containing steels conventionally used for cryogenic temperature structural applications, e.g., steels with nickel contents of greater than about 3 wt %, have low DBTTs, but also have relatively low tensile strengths. Typically, commercially available 3.5 wt % Ni, 5.5 wt % Ni, and 9 wt % Ni steels have DBTTs of about -100.degree. C. (-150.degree. F.), -155.degree. C. (-250.degree. F.), and -175.degree. C. (-280.degree. F.), respectively, and tensile strengths of up to about 485 MPa (70 ksi), 620 MPa (90 ksi), and 830 MPa (120 ksi), respectively. In order to achieve these combinations of strength and toughness, these steels generally undergo costly processing, e.g., double annealing treatment. In the case of cryogenic temperature applications, industry currently uses these commercial nickel-containing steels because of their good toughness at low temperatures, but must design around their relatively low tensile strengths. The designs generally require excessive steel thicknesses for load-bearing, cryogenic temperature applications. Thus, use of these nickel-containing steels in load-bearing, cryogenic temperature applications tends to be expensive due to the high cost of the steel combined with the steel thicknesses required.
Although some commercially available carbon steels have DBTTs as low as about -46.degree. C. (-50.degree. F.), carbon steels that are commonly used in construction of commercially available process components and containers for hydrocarbon and chemical processes do not have adequate toughness for use in cryogenic temperature conditions. Materials with better cryogenic temperature toughness than carbon steel, e.g., the above-mentioned commercial nickel-containing steels (3 1/2 wt % Ni to 9 wt % Ni), aluminum (Al-5083 or Al-5085), or stainless steel are traditionally used to construct commercially available process components and containers that are subject to cryogenic temperature conditions. Also, specialty materials such as titanium alloys and special epoxy-impregnated woven fiberglass composites are sometimes used. However, process components, containers, and/or pipes constructed from these materials often have increased wall thicknesses to provide the required strength. This adds weight to the components and containers which must be supported and/or transported, often at significant added cost to a project. Additionally, these materials tend to be more expensive than standard carbon steels. The added cost for support and transport of the thick-walled components and containers combined with the increased cost of the material for construction tends to decrease the economic attractiveness of projects.
A need exists for process components and containers suitable for economically containing and transporting cryogenic temperature fluids. A need also exists for pipes suitable for economically containing and transporting cryogenic temperature fluids.
Consequently, the primary object of the present invention is to provide process components and containers suitable for economically containing and transporting cryogenic temperature fluids and to provide pipes suitable for economically containing and transporting cryogenic temperature fluids. Another object of the present invention is to provide such process components, containers, and pipes that are constructed from materials having both adequate strength and fracture toughness to contain pressurized cryogenic temperature fluids.